18 hours fermentation of Biga

1: Done is when you are happy with the result, less than 18 hours means less fermentation, more means more fermentation, it all depends on how much flavor you want to get out of it. 18 hours is approximate and as you say there are many factors. For a biga I'd use my nose, you will get a feel for when it is ready. Much more than 18 hours might get a bit strong.

2: Refrigerators are great tools for bread baking because they slow down the processes without stopping them or killing anything. You can ferment your biga/starter, or prove your bread in the fridge, it just takes longer. You could leave it out for 10 hours and then 16 in the fridge for example, or start it 2 days early and keep it in the fridge the whole time. A refrigerator is also very consistent in temperature, whereas room temperature varies from day to day

Keep in mind that the starter fermentation is about flavor development, and is an optional step. I always use a starter (a starter is a biga you don't ferment, you use it to get a good, healthy base for the rest of the dough) when making pizza dough but I rarely ferment it and I still get excellent results. Often I will make the dough and then refrigerate it for a day, I get the flavor development later in the process. So if you have 6 hours you can ferment the biga do it for 6 hours, it's not critical.

I pretty much agree with most of GdD's answer, but I'll just add that this particular biga formula is already going to be problematic in terms of exactness, due to the fact that it requires 0.14 gram of yeast or 1/24th of a teaspoon. That's a small enough quantity of yeast that even a few more or a few less granules could make a significant difference in how much yeast grows in total over 18 hours, and the freshness of the yeast will come into play at that point too, as well as the type of yeast. All of those factors could be just as significant with this tiny quantity of yeast as a variation in a few degrees of room temperature, etc.

Basically, it's an inexact formula. If 12 hours is more convenient, try that. If 24 hours is more convenient, try that. Perhaps use a tiny bit more or a tiny bit less of this miniscule amount of yeast when you make the variations, if you want. If it's hot in your kitchen, ferment it for a shorter amount of time; if it's cold, do longer. Since the final dough linked in the question adds a bunch of yeast, the actual final count of yeast in the biga doesn't matter much. What you're really doing is letting the dough sit at room temperature not only for yeast growth, but also for enzymes to work within the dough for flavor development, etc. This particular biga formula appears to be mostly about flavor, not about affecting the final rise of your bread.

Given the tiny amount of yeast, I personally wouldn't recommend trying to do the entire biga fermentation in the fridge, as you likely won't get much yeast growth at all. You'll still get some enzymes (and some other things growing in a dough, like lactic bacteria after 18+ hours, which can also give flavor), but the point of the biga at room temperature seems to give some "yeasty" notes, which will only happen if you let this tiny amount of yeast multiply at least a bit. But as long as you're fermenting it for several hours before refrigeration, it will still provide a lot of the flavor you're likely looking for.